1. Field of the Invention
This invention generally relates to video terminal devices and more specifically to a circuit for controlling the scan to a display device.
2. Related Inventions
This invention is used in conjuction with the invention described in Ser. No. 516,348, filed on Oct. 21, 1974, by Michael D. Morganstern et al. and assigned to the same assignee as the present invention.
3. Description of the Prior Art
In conventional television circuits, a cathode ray tube has a series of dot patterns displayed on its screen which form either alphanumeric characters or pictures. The display is formed by a beam which horizontally scans the entire surface of the cathode ray tube and an electronic gun in the neck of the cathode ray tube which shoots out a series of dots during each horizontal scan.
The scan consists of a forward and backward portion with part of the forward portion being used for displaying the dots from the electronic gun. Vertical adjustment such that each forward scan does not occur in the same location is also provided. When all the forward horizontal scans for the screen have been displayed, the scan is set back to its original, i.e. upper right, position such that another viewing of the dot pattern is provided. This happens at fast speeds as is apparent by the fact that it is impossible to see the dot patterns as they are forming the alphanumeric characters or pictures on the cathode ray tube screen.
It has been found worthwhile in the computer industry to utilize the cathode ray tube as a display device for reading out computer information. However, not all of the television technology related to the control of the scan is desirable or applicable. Thus, in the television industry, a free-running oscillator is synchronized to a composite signal generated from the transmitter of the television station to provide for the display on the screen. In a computer, however, no composite video signal is ever provided. Moreover, it is desirable to obviate other circuitry used in the television industry.
The present invention eliminates the free-running oscillator and the composite video signal of the television industry. It does this by making use of a standard digital crystal clock oscillator which provides precisely timed pulses. This crystal clock oscillator is used to control the timing of the scan as well as the timing of the video signal applied to the scan.
Another deterrence for the computer industry in using television circuitry is that the television industry overscans the display on the cathode ray tube screen. By overscanning is meant that the beam is displayed beyond the extremities of the screen. With part of the picture being off the screen, a percentage of the information which has been transmitted to the sceen is lost. While this is immaterial in the television industry since the essential part of the picture is captured on the visible portion, it is unacceptable in the computer industry since the time needed to display the information is severely limited.
For example, in computer industry the typical number of characters displayed per horizontal line is 180. This means that the cathode ray tube must have 80 characters provided to it within the visible (usable) portion of the forward scan of the beam. Since the time for a scan cycle is fixed, the time when this information is able to be displayed must be maximized. Thus, if overscanning is provided, a lesser percentage of the forward scan time is usable to present the characters on the screen. This time, in the video terminal application means that a faster character generator to process the characters to be displayed on the video screen is necessary with the result that the character generator would be much more expensive and beyond the state of present high yield technology.
In order to maximize the display time, the computer industry utilizes underscanning. As used herein, underscanning means that the scan is regulated such that it does not extend beyond the boundaries of the screen. Obviously, this also allows the character generator providing the display characters to be slower and consequently cheaper. However, this requires precise control of the time between the start of the display of information and the movement of the beam.
One of the specific problems involved in increasing the time to display characters on a screen involves the control of a power transistor which initiates retracing of the beam. In the television industry, the power transistor is oversaturated, i.e., it is driven as much as possible, so as to be sufficient for the worst case gain transistor. This results in a high desaturation time and the expense of extra power supplied to the power transistor. Because of the parameters involved in the television industry, both overscanning and extra power are permissible. However, in the computer art, underscanning is required and retrace time becomes critical. As a result, the oscillator and phase lock loop must be eliminated while the timing of scan and the scan's overall regulation must be controlled so that the first character appears at the proper place in the forward portion of the scan.